This invention pertains to the field of particle size measurement of industrial particles and more specifically to the on-line measurement of the particle size distribution (PSD) of particles in a liquid suspension. By suspension is meant a solid or liquid discrete particle in a liquid carrier or matrix. Examples of suspensions of interest are a slurry (a high concentration of more than about 10% to 15% solid particles by volume in a liquid), a dispersion (a low concentration of less than about 10% to 15% solid particles by volume in a liquid), and an emulsion (liquid particles or droplets in a liquid). It relates to the measurement of PSD of sub-micron sized particles in a suspension, and in systems with particle sizes larger than 1 micron. It also relates to systems used to determine the concentration and the degree of agglomeration in multiphase systems. It relates to systems useful in various industrial applications including emulsification (droplet size), homogenization (quality of dispersion), grinding (particle size distribution), precipitation of metals (particle agglomeration), and on-line measurement of particle formation (particle size distribution).
It is known that the frequency-dependent attenuation of ultrasound in suspensions is determined by the PSD within those systems (the “Forward Problem”). Several theoretical models have been developed to treat the absorption of the ultrasound for a variety of systems. In particular, Allegra and Hawley [Attenuation of Sound in Suspensions and Emulsions: Theory and Experiments. J. Acoust. Soc. Am. 51 (1972) 1545-1564] provide a mathematical framework for calculating the attenuation of ultrasound in dispersions and emulsions. The Allegra-Hawley model is completely general, allowing one to calculate the absorption for new systems without having to develop new models. Although this theory is for monodisperse (single-sized) suspensions at low volume concentrations (20% or less), it is easily extended to polydisperse systems by integrating the calculated absorption over the PSD density function.
Holmes and Challis [Ultrasonic Scattering in Concentrated Colloidal Suspensions. A. K. Holmes, R. E. Challis in Colloids and Surfaces A: Physicochemical and Engineering Aspects 77 (1993) 65-74; and A Wide Bandwidth Study of Ultrasound Velocity and Attenuation in Suspensions: Comparison of Theory with Experimental Measurements. A. K. Holmes, R. E. Challis, D. J. Wedlock in J. Colloid Interface Sci. 156 (1993) 261-268] have measured absorption and phase velocity in monodisperse polystyrene suspensions of up to 45% volume fraction and found good agreement with the predictions of single and multiple scattering models [Multiple Scattering of Waves. P. C. Waterman, R. Truell in J. Math. Phys. 2 (1961) 512-540, and Wave Propagation Through an Assembly of Spheres IV: Relations Between Different Multiple Scattering Theories. P. Lloyd, M. V. Berry in Proc. Phys. Soc. 91 (1967) 678-688]. Holmes and Challis use a wide-band pulse combined with a high-speed digitizer and a Fourier Transform operation to acquire the ultrasonic spectrum. They use a pair of transducers at a fixed separation for an off-line, through-transmission measurement, and ignore all but the primary transmitted pulse. They do not demonstrate the ability to measure PSD with their apparatus.
Since a known PSD can be used to predict the absorption as a function of frequency, it should also be possible to invert ultrasonic spectra to predict the PSD based only on the absorption (“the Inverse Problem”). It turns out that inverting the ultrasonic data is an art in itself, as variations in this data cause instability in the standard inversion methods.
Much of the ultrasonic work to date has been concerned with measuring the size of relatively coarse particles (>10 microns). One of the first ultrasonic-based instruments was the Armco Autometrics® PSM-400 [Particle Size and Percent Solids Monitor. C. Cushman, J. Hale, V. Anderson in U.S. Pat. No. 3,779,070 (1973)], used to control grinding circuits in the mineral industry. It used narrowband, stationary transducer pairs (each pair operating at a single fixed frequency) and a semi-empirical model to provide an indication of median particle size (max. 600 micron).
U.S. Pat. No. 3,779,070 to Cushman et al in FIGS. 37 to 39 shows an arrangement of separate sending and receiving ultrasonic transducers on opposite sides of a ore slurry flow passage or transducers (which both send and receive) on one side and an ultrasound reflector on the opposite side. The transducers are in direct contact with the slurry. Slurries with mean particle diameters of from 40 to 250 microns are discussed. Separation of the transducers is about 4.0 inches (10.2 cm). The flow passage is placed directly in a container (sump) of slurry or a slurry pipeline. One or two pairs of transducers may be used. When two transducer pairs are used, each operates alternately, one to determine particle size and the other to determine percent solids in essentially the same volume of the slurry. Two different ultrasonic frequencies may be used depending on the attenuation expected from the sample, but once a frequency is selected it remains constant. Selected frequencies may be within the range of about 0.3 to 3.0 MHz for particle size distributions with a median size of about 150 microns or smaller. In cases where a single pair of transducers are used (in a system using a reflector), the two transducers operate alternately at two different frequencies. The ultrasonic particle size measuring system provides real-time results and may be part of a feedback loop for automatic control of a circuit for ore grinding. Two major limitations are that any variations in the concentrations must be known and the size distribution is not measured with this method.
Riebel and Loffler [The Fundamentals of Particle Size Analysis by Ultrasonic Spectrometry. Part. Part. Syst. Charact. 6 (1989) 135-143] obtain an acoustic attenuation spectrum (2-80 MHz) with one pair of wideband transducers to infer the entire PSD for particles ranging between 20 and 1000 microns. Their physical model is based on the Lambert-Beer law (max. concentration 10% vol) and the assumption that the particle size-dependent attenuation at each applied frequency is proportional to the total particle surface encountered by the sound wave traversing the medium. This assumption is valid only in the short wavelength limit.
U.S. Pat. No. 4,706,509 (1987) to Riebel discloses using ultrasound for sampling multiple particle size intervals to determine a particle size distribution preferably with 5 or more particle size intervals. A number of different discrete ultrasonic frequencies are successively passed as tone bursts through a suspension of particles in a liquid; preferably the number of frequencies equals the number of intervals sampled. One or more pairs of ultrasonic transmitters and receivers may be used for through transmission measurements, or the same transducers can serve as both a transmitter and receiver of the echo resulting from an opposed reflector. If a plurality of ultrasonic wave transmitters are excited continuously standing waves are avoided by arranging the absorption path at an angle other than 90 degrees to the walls of the suspension enclosure. Preferably, the frequencies selected for excitation are such that the wavelength corresponding to the lowest frequency is greater than the diameter of the largest particles to be expected, and the wavelength corresponding to the highest frequency is less than the diameter of the smallest particles to be expected. The frequency range contemplated by Riebel would therefore limit his method to particles larger than 15 microns. Extensive calibration of the system is required, and changes in particle morphology have been observed to require a new calibration.
Recent work has tried to extend ultrasonic-based measurements to cover the sub-micron particle size regime. Pendse and Sharma [Particle Size Distribution Analysis of Industrial Colloidal Slurries Using Ultrasonic Spectroscopy. Part. Part. Syst. Charact. 10 (1993) 229-233] report on a prototype instrument named the AcoustoPhor® (Pen Kem 8000) which comes in both off-line and on-line versions. With this system the acoustic attenuation is measured at several discrete frequencies between 1 and 100 MHz. At these frequencies viscous energy dissipation of the sound wave is the dominant phenomenon for sub-micron, rigid particles. These authors Claim to be able to determine the PSD in the range of 0.01 to 100 microns for slurry concentrations as high as 50% (vol).
U.S. Pat. No. 5,121,629 (1992) to Alba uses the more general Allegra-Hawley model (which includes viscous, thermal, and scattering components) as the basis for an off-line instrument (Malvern UltraSizer) that measures PSD in the range of 0.01 to 100 microns for slurries with concentrations up to 50% (vol). In this patent, Alba uses two wideband ultrasonic transmitters and receivers working at selected, discrete frequencies within the 0.5 to 100 MHz range to determine particle size distribution and concentration for sizes smaller than a micron and concentrations higher than 15% by volume. He suggests that different embodiments could use arrangements like pulse-echo, tone burst transmission/detection, or multiple transducers. Off-line calculated attenuation spectra are compared to on-line measured attenuation spectra of the test sample to rapidly estimate the size distribution and concentration via a fitting routine. He suggests the method is applicable to both off-line and on-line operation, but it has been observed that the preferred embodiment of the instrument requires 4-5 minutes to collect and process the data for the discrete frequencies as taught. This system is inappropriate for in-line measurements on rapidly flowing suspensions or suspensions undergoing rapid changes.
U.S. Pat. No. 3,802,271 to Bertelson uses an acoustic signal to analyze particles in a fluid, preferably combustion dust particles in industrial smokestack emissions. He avoids the need to scan frequencies by employing a complex wave shape of a non-sinusoidal waveform comprehending several frequency components. Contemplated are rectangular, square, or sawtooth waveforms that are generated by a variable frequency oscillator that drives a speaker directly or through a pulse generator. The apparatus is said to be useful by using either a frequency scan or a single non-sinusoidal waveform. The method for analyzing particles includes the step of generating sound wave energy having plural frequency components and transmitting it through the fluid with and without the particles. A major limitation is his requirement of a physical means to separate particles from the fluid so as to provide an acoustic path without particles.
U.S. Pat. No. 5,831,150 to Sowerby et al uses a plurality of ultrasonic beams with discrete frequencies to measure particle sizes in the sub-micron range, such as TiO2 particles in paint samples. Solid content, however, was at a low percentage, such as 2.3%. One embodiment of the invention uses six pairs of piezoelectric transducers to transmit and receive ultrasound of specific frequency. Alternatively, fewer wideband ultrasonic transducers can be used to generate the tone bursts. In all of the disclosed embodiments, the PSD is estimated from the ultrasonic phase velocity. A limitation is the use of a radioactive density gauge to measure the concentration, which is used as an input to the ultrasonic sensor.
The above instruments, in their preferred embodiments, are similar in that they measure the attenuation spectrum one frequency at a time, using either swept-frequency (“chirp”) generators or a series of tone-bursts. That approach works well in the laboratory, but it is comparatively slow at collecting data. For on-line application, the sample in the flow cell is changing as the data is collected; consequently, the upper and lower frequency components measured at different times do not relate to the same moving physical particles. Other instruments that use so-called “pulse” generators actually produce a pulse train, which is equivalent to a tone burst (essentially a single frequency or a narrow band of frequencies); such instruments must generate successive tone bursts at several frequencies to measure the attenuation spectrum.
The choice of frequencies in the 1-100 MHz range taught by the prior art imposes a severe restriction on the maximum separation between the transducers: for a suspension having a high concentration of sub-micron particles (10%-50%), the attenuation is so high in that range that the maximum gap can be only a fraction of an inch (typically 0.05-0.10 inch, 0.13-0.26 cm). Small transducer gaps are not well suited for on-line applications, since such small clearances tend to become plugged. Milling operations such as media mills and attritors are designed to reduce the particle size in solid/liquid slurries. The slurries tend to be concentrated (ranging from 20-50% solids by volume), and in the case of sub-micron particles there are no in-line commercial instruments that can measure either particle size or particle size distribution (PSD) of undiluted suspension without becoming plugged after brief operation.
In order to invert spectral data into PSD, most of the prior art (except Alba and Sowerby) uses a phenomenological model as opposed to a physical model. Therefore the task of switching a particular instrument from one process stream to another requires new calibration curves to be developed.
There is a need for a method for obtaining particle size distribution and concentration that allows faster data acquisition at a lower system cost than previous instruments. There is a need for a system that can operate in an industrial environment and obtain on-line results with robust, reliable performance that requires minimal maintenance and avoids a narrow transducer gap prone to plugging. There is a need for a system to invert ultrasonic spectral data into PSD based on a physical model where switching the instrument from one process stream to another requires only substituting the appropriate physical constants.
In the production of precipitated particles for certain applications, there is a need to monitor variations in an aging master batch of solution so that sub-batches can be withdrawn at appropriate times to produce particles of the correct size when combined with a reducing solution that causes precipitation. U.S. Pat. No. 5,389,122 to Glicksman teaches a system for preparing finely divided, spherical shaped silver particles (typically 1-3 microns) using a chemical aging and precipitation process. In the master, the particle size is changing during the aging process, and any sample pulled would still be so reactive that an accurate particle size could not be obtained. There is a need for a PSD system using a simple probe that can rapidly and accurately predict particle size and interact with an automated control system to regulate the addition of a material (such as a reducing agent) to form a finely divided, spherical particle size in a stirred tank system.